1. Field of the Invention
The present invention relates to an apparatus for determining concentrations of light absorbing materials in a living tissue by the utilization of intensities of light transmitted through or light reflected from the living tissue.
2. Related Art
A pulse oximetry is known as one of this type of instrument. The pulse oximetry noninvasively measures an oxygen saturation in arterial blood by the utilization of pulsations of light transmitted through a living tissue. To this end, the oximetry irradiates a living tissue with light of two wavelengths to produce pulsations of light L1 and L2; obtains changes .DELTA.A1 and .DELTA.A2 of the optical densities of the living tissue by the utilization of the pulsations of light; and computes an arterial oxygen saturation SaO.sub.2. Before computing the SaO.sub.2 from the .DELTA.A1 and .DELTA.A2, .PHI.12=.DELTA.A1/.DELTA.A2 must be computed. Conversion of .PHI.12 into SaO.sub.2 is carried out by use of a relationship between .PHI.12 and SaO.sub.2 of a human body, which are actually measured. The principle of this method is applicable to the measurement of every kind of light absorbing material contained in arterial blood. This method, called a pulse photometry, is practically used for the measurement of a dye dilution curve.
A near-infrared spectrometry (NIRS) is also known as another example of this type of measuring technique. The NIRS noninvasively measures an average oxygen saturation in arterial blood and venous blood by use of a light transmitted through the living tissue. This measuring technique is also applied to the method of measuring light absorbing materials in any of the other tissues than the blood. Example of those light absorbing materials are cytochrome and myoglobin. The near-infrared spectrometry produces an intended value from a received light by substituting measured intensities of light of wavelengths for a theoretical formula of light scattering Various theoretical formulae have been proposed.
Errors inevitably occur in noninvasively measuring a ratio of concentrations of light absorbing materials in a living tissue or in blood. Many causes for the error are known. In the case of the pulse oximetry, there exist many light absorbing materials and the pulsation of the living tissue exists as well as SaO.sub.2 as an object to be measured, those factors will cause errors in the SaO.sub.2 measurement.
Where an attempt is made to remove the adverse effects by those factors and to improve a measurement accuracy, the necessity is to increase the number of wavelengths of light used and the number of the related formulae, and to arrange those formulae into simultaneous equations and to solve the resultant.
The blood as one of the key light absorbing materials as well as the tissue, exhibits a nature of light scattering. Many factors are involved in the optical measurement of scattering material. Light tends to scatter in a short wavelength region. Then, when incident and collimated light rays propagate through the tissue, scattering of light gradually grows and the propagating paths of light are different with their wavelengths This fact is essential to secure an improved accuracy of measurement, and requires complicated numerical correction.
A variety of methods have been used for the near-infrared spectrometry (NIRS), but reliable methods have never been presented so far as we know. This fact shows an intricacy of the problem by the scattering nature.